302 Journal of Applied Microscopy. 



Hygiene of Harvard Medical School. The difference in percentage composition 

 lies in the second place of decimals, and is therefore of little moment. 



Ever}^ step given in the table (except Step No. 4) is necessary to exact 

 uniformity in results. For instance, if Step No. 11 be omitted, the reaction 

 may be adjusted to -1.5 accurately enough, so that the acid contents of various 

 successive lots of media will be uniform, but the degree of concentration of 

 the other constituents, depending as it does on the loss through evaporation 

 during Step No. 10, is almost certain to vary from lot to lot, for the extent 

 of the surface of media exposed to the air affects the amount lost. Thus 500 cc. 

 of media will lose more if cooked in a wide, shallow vessel, than it will lose in a 

 narrow, deep vessel. Moreover, a greater percentage of loss by evaporation 

 occurs in preparing 500 cc. of media in one vessel than in preparing 2000 cc. of 

 the same media in the same vessel. 



In Step No. 16, it is to be noted that the cotton itself filters out only the 

 bulky coagulum formed in the cooking processes, No. 10 and No. 14. This 

 bulky coagulum, however, once deposited on the cotton, forms thereafter an 

 extremely good filtering material, and ensures a perfectly clear filtrate, if the 

 filtrate be passed through it two or three times. 



Step No. 15 is necessary, because a reaction adjusted to -1.5 per cent, will 

 rise if the media be subsequently concentrated. It is to be noted, however, 

 that in adjusting the reaction to +1.5 per cent, by Fuller's method (addition of 

 normal acid solution) a certain quantity of water is added at the same time — the 

 water of the normal solution — so that the five minutes boiling (Step No. 14) 

 usually brings the final weight of the finished medium to about the original 

 weight of the infusion. 



The writer holds strongly the view, based upon comparative bacterial counts, 

 that the practice of sterilizing media in bulk, with the object of subsequent 

 successive removal and use of small portions, is to be condemned. The media 

 in bulk, left after the withdrawal of these portions, generally requires further 

 sterilization after each withdrawal. The portion withdrawn also generally 

 requires sterilization. Hence the later portions withdrawn receive an amount of 

 sterilization not usually given to media tubed and sterilized directly after prepara- 

 tion. Additional sterilization means really additional cooking. If one lot of 

 media be cooked twice as long as another, there can be little doubt that the 

 composition of the two lots will differ somewhat. It is true that the exact series 

 of changes which take place amongst the many delicate chemical compounds 

 present in meat extracts during the preparation of media are not known. On 

 the other hand, a rigid uniformity of technique will ensure that, as far as pos- 

 sible, the same series of changes (whatever the changes may be) will be 

 obtained in every lot of media prepared. This is as far as any one can go at the 

 present time. No one should be content with anything short of this. 



In sterilization, therefore, an exact degree and length of time of exposure to 

 heat should be adopted. Fifteen minutes in flowing steam on three successive 

 days will sterilize any liquid or liquified media in tubes. More than this is 

 unnecessary, except where media in bulk are to be sterilized. Prolonged heating 

 is then necessary to ensure the proper temperature throughout the whole mass. 



